1. Field of the Invention
The present invention generally relates to a silver powder and a method for producing the same. More specifically, the invention relates to a silver powder for a conductive paste for use in electronic parts, such as internal electrodes of multilayer capacitors, conductive patterns of circuit boards, and electrodes and circuits of substrates for plasma display panels, and a method for producing the same.
2. Description of the Prior Art
As a conventional conductive paste for use in electronic parts, such as internal electrodes of multilayer capacitors, conductive patterns of circuit boards, and electrodes and circuits of substrates for plasma display panels (PDP), there is used a silver paste produced by mixing a silver powder and a glass frit in an organic vehicle and kneading them. As the size of such electronic parts decreases, it is required that a silver powder for a conductive paste has reasonably small particle diameters and a reasonably narrow range of particle diameters in order to form a conductive pattern or the like having a high density and fine lines.
As a method for producing such a silver powder for a conductive paste, there is known a method for adding an alkali or a complexing agent to an aqueous silver salt containing solution to form a silver oxide containing slurry or an aqueous silver complex salt containing solution, and thereafter, adding a reducing agent to the silver oxide containing slurry or the aqueous silver complex salt containing solution to deposit a silver powder by reduction.
However, the particles of the silver powder produced by such a conventional method violently cohere, so that there is a problem in that the silver powder can not be applied to recent electronic parts having fine lines, such as internal electrodes of multilayer capacitors, conductive patterns of circuit boards, and electrodes and circuits of substrates for plasma display panels.
Thus, in order to form a silver powder which includes a smaller number of particles easy to cohere and which has excellent dispersibility, there has been proposed a method comprising the steps of: adding an alkali or a complexing agent to an aqueous silver salt containing solution to form a silver oxide containing slurry or an aqueous silver complex salt containing solution; adding a reducing agent thereto to deposit silver particles by reduction; and thereafter, adding at least one of fatty acids, fatty acid salts, surface active agents, organic metals and protective colloids, serving as a dispersing agent to the silver containing slurry solution to form a silver powder (see, e.g., Japanese Patent Laid-Open No. 10-88206).
However, if the silver powder formed by adding a fatty acid, a surface active agent or the like as a dispersing agent as described in Japanese Patent Laid-Open No. 10-88206 is used for preparing a paste, there are some cases where the paste causes phase separation to form suspended matters. The suspended matters form pinholes in the film of the paste to decrease the density of the film after burning.
If a paste prepared by using the silver powder formed by adding a fatty acid, a surface active agent or the like as a dispersing agent as described in Japanese Patent Laid-Open No. 10-88206 is printed on a substrate, there are some cases where the paste is not easily released from a screen plate, so that bubbles are drawn into the paste. As a result, there are some cases where the thickness of the burned film is not uniform, so that the value of resistance of the film varies. Therefore, such a paste is unsuitable for a conductive pattern.
In particular, in a conductive paste for use in an electrode of a substrate for a plasma display panel, which is required to have a high density and fine lines, the decrease of the density of the film causes severe problems.
In addition, as a conductive paste used for forming an electrode of a substrate for a plasma display panel, an alkali developable photosensitive paste is ordinarily used. The alkali developable photosensitive paste generally comprises an alkali soluble polymer, a polymerizable monomer, a photopolymerization initiator, a solvent, a glass frit and a stabilizer. However, the alkali soluble polymer contains a carboxyl group to have a high polarity, so that there are problems in that bubbles are easily drawn into the paste during the screen printing, and the thickness of the burned film is not uniform.